A frequently-encountered object evaluation arises in the course of drawing operations or uncoiling of stock for final cutting and finishing.
In the course of such drawing or uncoiling, need exists for performing a "last chance" eddy current nondestructive testing (NDT) inspection for short defects by what is termed the encircling coil method, i.e., wherein a detection coil fully encircles the transported stock. The test method relies on either a set of high pass or band pass filters to maximize the signal-to-noise ratio of the apparatus output signal for any flaws encountered and the method is extremely speed sensitive.
Since final automatic sorting or grading decisions are made primarily on the basis of output amplitude sensitive threshold levels, a given defect indication must yield a fairly constant output amplitude and signal-to-noise ratio and difficulties accordingly attend the situation at hand. Thus, in the type of operations under discussion, the line speed is variable, alternately stopping or slowing and then accelerating rapidly to some maximum speed to complete the cycle.
It is well-known in eddy current NDT practices, where line speed is constant, to provide a plurality of filters appropriate for different constant speeds and for the operator to select the applicable filter by manual input to the apparatus. As speed varies, however, the selected filter becomes inadequate since the output amplitude and the signal-to-noise ratio vary excessively as the object traverses the test coil at different speeds.
Efforts have been also been addressed to automated change of filters responsively to the sensing of product transport speed, but those known to applicants herein simply endeavor to correlate speed change with filter choice over filters available in quite limited number, with the result that filter shifting is a discontinuous or step-like change, yielding less than optimal accuracy in fault detection.